The present invention relates to an optical module for optical height measurement that optically measures a surface height of a layer-structured specimen including a transparent or semi-transparent thin film.
There has been known a focus control signal used for an optical disk device that is used to highly accurately focus an optical disk as a specimen. Japanese Patent Application Laid-Open No. 9-265722 discloses a state in which the focus control signal changes according to a change in shift amount of a focal position to the specimen.
Japanese Patent Application Laid-Open No. 2008-102998 discloses an optical system for a multilayer optical disk that avoids a stray light from another layer and does not give an influence to a focus control signal, and discloses that the focus control signal can be generated also for a specimen having a layer structure similarly to Japanese Patent Application Laid-Open No. 9-265722.
While in Japanese Patent Application Laid-Open No. 9-265722, a light reflected by a specimen is guided to an optical detector by an objective lens and an objective lens group, generally, the objective lens group includes, for example, collimator lenses and cylindrical lenses, and the optical detector is achieved by the use of a four-divided detector. The cylindrical lens allows adding an astigmatism to the light. When a height of the specimen changes, a spot shape on the optical detector changes into an ellipsoid. Operating a signal change from the optical detector ensures generating a focus error signal correlated to a height.
However, in the case where the specimen has a layer structure including a semi-transparent thin film, a reflected light from the specimen surface interferes with a reflected light from the layered structure, possibly failing to generate the normal focus error signal.
Japanese Patent Application Laid-Open No. 2008-102998 discloses a method that receives only a light from a target layer and does not receive a stray light from another layer in an optical disk including a multilayer structure by a knife-edge method. The method for avoiding the stray light from another layer used in the optical disk is effective when a layer interval is sufficiently wide with respect to a focus detection range. However, in the case where the layered structure is not a normalized layered structure like an optical disk but is any given layered structure, there may be a case where entering of the stray light from the other layer into a light-receiving surface causes a failure to generate a normal focus error signal.
In the case of a detection system that uses a method of receiving only the light from the target layer and does not receive the stray light from the other layer, the focus detection range needs to be sufficiently small with respect to the layer interval.